Basic Buck-boost Converter Schematic

Step-up/down (Buck-boost) Converters

A Buck-boost converter is used in applications where input
voltage can vary, either below or above the output voltage (i.e. when using
batteries).

The four internal MOSFET switches will be automatically configured to form
a Buck when VIN exceeds VOUT and change to Boost operation
when VIN falls below VOUT. This makes Buck-boost ideal
for battery-powered
applications to extend battery life when the battery voltage falls
below the regulated output voltage value.

Due to the fully synchronous operation, the efficiency of the Buck-boost
converter is very good. The output current capability in Buck mode is normally
higher than in Boost mode, due to step-up requiring higher switch current at
the same load conditions when compared to Buck mode.

The MOSFET voltage ratings will determine the maximum input and output voltage
range.

In applications where the output voltage does not need to be referenced to
ground (like LED drivers), buck-boost converters with a single switch and rectifier
are used. In most cases, the output voltage is then referenced to VIN.